Television receivers are predominantly located in the home, and this fact limits the practical size of most receivers, since they can be sized no greater than may conveniently pass through an accessway, such as the front or rear door of the home. Within such modern television receivers, signal processing designs and systems have been implemented which are capable of providing a defect-free, high resolution picture image on the display screen within a reasonable viewing angle, such as 45.degree.. With these known techniques which include adaptive comb filter separation of chrominance and luminance, video transition crispening and line doubling with adaptive interpolation, a conventionally formatted television picture signal (e.g. NTSC or PAL) will be displayed without any apparent loss or lack of resolution or artifacts other than random noise.
Thus, random noise becomes the limiting factor in picture quality. With comb filter signal processes and crispening processes, random noise takes on a certain coherence or lack of randomness. In these processes signal components in the vertical and horizontal direction are favored over components along 45.degree. diagonals. This favoritism leads to a coherence which is manifested as the transformation of a randomly shaped noise blob or impulse into a box-like image having squared dimensional characteristics in the picture display. While the resultant box-like image resembles film grain, it is not such, and it can prove to be annoying when it reaches the level of conscious perception on the part of the viewer.
A number of encode-decode noise reduction processes have been proposed wherein preemphasis has been added to low picture level details at the transmit end of the transmission path in order to enable more ready separation of such low level details from transmission path added noise at the receiver. One approach is described in the present inventor's U.S. Pat. No. 4,837,611 entitled "Wideband NTSC-Compatible Transmission System with Noise Reduction Processing" which employs two adjacent television channels, and which is particularly well suited to cable television. Another approach is described in the present inventor's U.S. Pat. No. 4,918,515 entitled "Compatible Television Transmission System with Separate Preemphasis Path Within a Single Channel". Each of these two exemplary approaches to noise reduction requires a modification of the conventional broadcast television signal format standards for operability, a subject involving both technical and political issues.
Thus, despite the innovations identified above, a need has remained for a more universally acceptable noise reduction system which requires no modification of the existing television signal format standards and which may be used in all television receivers. That is to say, the noise reduction system is single ended, with all of the processes occurring at the television receiver.
Single ended noise reduction techniques are not new. The present inventor has been active in this field, as shown by his prior U.S. Pat. Nos. 4,443,817; 4,670,775; and 4,672,429, for example. However, these prior approaches do not fully intermingle or mesh together the information available in the horizontal, vertical and temporal domains in order to achieve a more effective single ended noise reduction system.
There are two fundamental approaches to noise reduction at a television receiver. The first approach is known as "coring". The second approach employs temporal recirculation techniques. Each has its own advantages and drawbacks.
Coring processes provide the selective insertion of a low pass filter structure for low amplitude video transitions, whether they are horizontal, vertical or temporal. Coring has the advantage of relative simplicity and is usually carried out in the horizontal domain. More complex coring systems have been proposed for coring in the vertical and/or temporal domains, and for adaptive adjustment of the coring threshold, based upon motion content of the picture, for example.
Irrespective of the implementation details, the coring process operates upon only one characteristic of noise: its low amplitude; and, coring processing necessarily results in a lower frequency response for small amplitude signals in the picture display. The appearance of a cored picture image is unpleasant aesthetically. What is otherwise a grainy but high resolution picture becomes a grainless but very soft picture image as to low level details. For example, a picture image of a bookcase may have very sharp features relating to the horizontal and vertical edges of the bookcase and its shelves, whether or not coring is being done, since the edges are large transition level signals. However, without coring, the somewhat grainy picture will enable the viewer to read the titles on the books on the bookshelves, while when coring is inserted, the book titles become completely blurred and undecipherable by the viewer.
The temporal recirculation approach to noise reduction operates upon the principle of spreading out a noise impulse over a number of successive picture frames, thereby to reduce or minimize its visual impact. However, the recirculation technique is necessarily limited to stationary picture images. For evident reasons, recirculation of the picture image breaks down in the presence of motion. The amount of blurriness to be tolerated has generally been considered to be a function of the noise level. It is not uncommon to find television signal processing appliances, such as VCRs, having a manual control for selecting a recirculation threshold at 3 dB, 6 dB, or 12 dB, for example. The user is then free to select a compromise between blurring and noise, generally in function of the actual noise level then present.
Thus, a hitherto unsolved need has existed for a reception-end noise reduction system which more effectively reduces transmission path noise in a television picture signal.